The present application relates to nuclear magnetic resonance (NMR) imaging and, more particularly, to a novel method of, and apparatus for, performing slice selection while utilizing a switch-mode gradient power amplifier in the NMR system.
The use of linear gradient power amplifiers in NMR imaging systems is now well known. The present-day trend to high-speed NMR imaging places additional, and generally severe, demands upon existing linear gradient power amplifiers. These energy-inefficient linear amplifiers are also a main source of consumption of expensive electric power in a NMR imaging (MRI) installation. It has been suggested that a high-efficiency switch-mode power amplifier be used for providing each of the required gradient magnetic field signals. Several switch-mode gradient power amplifiers investigated show substantial promise for producing stronger and faster gradient magnetic fields, while consuming less power than linear gradient power amplifiers. However, one presently unacceptable problem with the use of switch-mode power amplifiers is the presence of a ripple at the amplifier switching frequency (e.g. often in the 10-500 kHz. frequency range) generally of about 1% to about 10% in amplitude, which ripple appears on the gradient waveforms, and hence in the gradient magnetic field. This gradient magnetic field ripple degrades the slice profiles produced by standard slice-selection pulses. Methods of, and apparatus for, at least reducing the effects of the switch-mode gradient power amplifier ripple, upon slice selection, is therefor highly desirable.